For me portal 3 will be a matchup of portal 3 and half-life episode 3,were chell finds a city and a metor falls into the city,it's the blue core(wheathly!)and he discover a way to control the aliens from half-life and takes chell to aperture laboratory's because aperture science have the boaralises wich contains a secret for the aliens to control the world,meanwhile in half-life episode 3 gordon freeman and Alyx start reserching for the aperture science labs to find the boat and stop the aliens to rule the world

Also i think there should be some co-op mission were you play as Chell or Gordon Freeman.